Epitaxial structure and epitaxial growth method for forming epitaxial layer with cavities

ABSTRACT

An epitaxial growth method includes the steps of: providing a substrate; forming a sacrifice layer on the substrate; patterning the sacrifice layer to form a plurality of bumps spaced apart from each other on the substrate; epitaxially forming a first epitaxial layer on the substrate to cover a portion of each of the bumps; removing the bumps to form a plurality of cavities; and epitaxially forming a second epitaxial layer on the first epitaxial layer such that the cavities are enclosed by the first epitaxial layer and the second epitaxial layer. An epitaxial structure grown by the method is disclosed as well.

RELATED APPLICATIONS

This application claims priority to Taiwan Application Serial Number101124450, filed Jul. 6, 2012, which is herein incorporated byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to an epitaxial structures and anepitaxial growth method for forming an epitaxial layer with cavities.

2. Description of Related Art

There has been rapid progress in technologies of light emitting diodes(LEDs) in recent years. For the purpose of increasing thelight-extraction ratio of LEDs, techniques such as patterned sapphiresubstrates have been widely applied in LEDs, which include galliumnitride. For further increasing the light-extraction ratio, it have beenpurposed to form cavities or pores in epitaxial gallium nitride layersof LEDs. In this regard, it is difficult to well control the shape andthe volume of the cavities in LEDs, and therefore the quality of LEDs isunstable in prior art. In view of the above, there exists a need of anew method that would improve the drawbacks in prior art.

SUMMARY

An epitaxial growth method for forming an epitaxial layer with cavitiesis provided. The method includes the steps of: providing a substrate;forming a sacrifice layer on the substrate; patterning the sacrificelayer to form a plurality of bumps spaced apart from each other on thesubstrate, wherein a portion of the substrate between the bumps isexposed; epitaxially forming a first epitaxial layer on the exposedportion of the substrate, wherein the first epitaxial layer covers aportion of each of the bumps, and a top surface of each of the bump isexposed; removing the bumps to form a plurality of cavities; andepitaxially forming a second epitaxial layer on the first epitaxiallayer such that the cavities are enclosed by the first epitaxial layerand the second epitaxial layer.

According to one embodiment of the present disclosure, the substrate isa sapphire substrate or a silicon substrate.

According to one embodiment of the present disclosure, the firstepitaxial layer comprises a group III nitride semiconductor.

According to one embodiment of the present disclosure, the secondepitaxial layer comprises a group III nitride semiconductor.

According to one embodiment of the present disclosure, each of the firstand the second epitaxial layers comprises gallium nitride.

According to one embodiment of the present disclosure, each of the firstand the second epitaxial layers is formed by a hydride vapor phaseepitaxy process, a metal organic chemical vapor deposition process or amolecular beam epitaxy process.

According to one embodiment of the present disclosure, a horizontalgrowth rate of the second epitaxial layer is greater than a horizontalgrowth rate of the first epitaxial layer.

According to one embodiment of the present disclosure, a temperature ofthe second epitaxial layer in the step of epitaxially forming the secondepitaxial layer is greater than a temperature of the first epitaxiallayer in the step of epitaxially forming the first epitaxial layer.

According to one embodiment of the present disclosure, a pressure in thestep of epitaxially forming the second epitaxial layer is less than apressure in the step of epitaxially forming the first epitaxial layer.

According to one embodiment of the present disclosure, the sacrificelayer is an inorganic material layer.

According to one embodiment of the present disclosure, the sacrificelayer comprises silicon oxide or silicon nitride.

According to one embodiment of the present disclosure, each of the bumpshas a maximum height of about 0.5 μm to about 3 μm, and each of thebumps has a bottom width of about 0.5 μm to about 5 μm.

According to one embodiment of the present disclosure, each of the bumpshas a taper angle of less than or equal to about 90 degrees in the stepof forming the bumps.

According to one embodiment of the present disclosure, the step ofpatterning the sacrifice layer comprises etching the sacrifice layer bya process of inductively coupled plasma reactive ion etching (ICP-RIE).

According to one embodiment of the present disclosure, each of the bumpsis removed by a wet etching process.

According to one embodiment of the present disclosure, an etchant of thewet etching process comprises ammonium fluoride (NH₄F) and hydrogenfluoride (HF).

According to one embodiment of the present disclosure, each of thecavities has an opening, and an area of the opening is about 5% to about50% of an bottom area of each of the cavities in the step of removingthe bumps to form the cavities.

According to one embodiment of the present disclosure, the substratecomprises a buffer layer formed thereon.

According to another aspect of the present disclosure, an epitaxialstructure is provided. The epitaxial structure includes a substrate, afirst epitaxial layer, a second epitaxial layer and a closed air void.The first epitaxial layer is disposed over the substrate. The secondepitaxial layer is disposed on the first epitaxial layer. The closed airvoid is embedded in the first epitaxial layer and the second epitaxiallayer. The closed air void has a bottom portion and a top portionrespectively formed in the first epitaxial layer and the secondepitaxial layer, and a width of the top portion is less than a width ofthe bottom portion.

In one embodiment of the present disclosure, the top portion of theclosed air void includes an arc surface, and the bottom portion of theclosed air void is a substantially flat surface.

In one embodiment of the present disclosure, the closed air void has across-section in a shape of trapezium.

In one embodiment of the present disclosure, the closed air voidincludes a stair-like sidewall, and the top portion and the bottomportion are two opposite flat surfaces.

In one embodiment of the present disclosure, the epitaxial structurefurther includes a buffer layer disposed between the substrate and thefirst epitaxial layer, and the bottom portion of the closed air void isformed on the buffer layer.

In one embodiment of the present disclosure, the epitaxial structurefurther includes a patterned sacrifice layer disposed on the substrate,and a top surface of the patterned sacrifice layer forms the bottomportion of the closed air void.

In one embodiment of the present disclosure, the closed air void has aheight of about 0.5 μm to about 3 μm, the bottom portion of the closedair void has a width of about 0.5 μm to about 5 μm.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the followingdetailed description of the embodiments, with reference made to theaccompanying drawings as follows:

FIG. 1 is a flow chart showing an epitaxial growth method according toone embodiment of the present disclosure;

FIG. 2A to FIG. 2F are cross-sectional views illustrating the processsteps of the epitaxial growth method 100;

FIG. 2G is a top view schematically illustrating the cavities formed instep 150 according to one embodiment of the present disclosure;

FIG. 3A to FIG. 3D are cross-sectional views illustrating the processsteps of an epitaxial growth method according to another embodiment ofthe present disclosure;

FIG. 3E to FIG. 3H are cross-sectional views illustrating the processsteps of an epitaxial growth method according to still anotherembodiment of the present disclosure;

FIG. 4A to 4F are cross-sectional views illustrating the process stepsof an epitaxial growth method according to still another embodiment ofthe present disclosure; and

FIGS. 5A and 5B are cross-sectional views illustrating the process stepsof an epitaxial growth method according to still another embodiment ofthe present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts. In thefollowing detailed description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the disclosed embodiments. However, it will beapparent, that one or more embodiments may be practiced without thesespecific details. In other instances, well-known structures and devicesare schematically shown in order to simplify the drawings.

FIG. 1 is a flow chart showing an epitaxial growth method 100 accordingto one embodiment of the present disclosure. The method 100 at leastincludes step 110 to step 160. FIG. 2A to FIG. 2F are cross-sectionalviews illustrating the process steps of the epitaxial growth method 100.The epitaxial growth method 100 disclosed herein may be applied inmanufacturing LEDs or flat display panels.

In step 110, a substrate 210 is provided, as depicted in FIG. 2A. Thesubstrate 210 may be a sapphire substrate, a silicon substrate or othersubstrates suitable for epitaxial growth.

In step 120, a sacrifice layer 220 is formed on the substrate 210, asdepicted in FIG. 2B. In one embodiment, the sacrifice layer 220 is alayer of inorganic material. The sacrifice layer 220 may compriseinorganic material such as silicon oxide or silicon nitride.

In step 130, the sacrifice layer 220 is patterned to form a plurality ofbumps 222, as depicted in FIG. 2C. The bumps 222 are spaced apart fromeach other on the substrate 210. The spacing interval between two bumps222 allows a surface of the substrate 210 to be exposed.

In one embodiment, a patterned photoresist layer is formed on thesacrifice layer 220, in which the photoresist layer covers the regionsthat are desired to form bumps 222, and the other portions of thesacrifice layer 220 is exposed. Thereafter, etching processes may beemployed to remove the exposed portion of the sacrifice layer 220, andthereby forming the bumps 222. Specifically, the pattern of thephotoresist layer dominates the contour of each of the bumps 222 in topview. Regarding the cross-sectional shape of each bump 222, it may bewell controlled by suitable etching techniques. In one example, theexposed portion of the sacrifice layer 220 is etched by the technique ofinductively coupled plasma reactive ion etching (ICP-RIE). The ICP-RIEtechnique may simultaneous provide anisotropic etching and isotropicetching, respectively contributed by ion bump and reactive ions, andtherefore each bump 222 may be formed in a shape of hemisphere. Theetching rates associated with the ion bump and the reactive ion may berespectively controlled by modulating the process parameters, and thus avariety of cross-sectional shapes of the bumps 222 may be formed. Forinstance, each bump 222 may have a cross-section in a shape of trapeziumor rectangle, which are described in detail hereinafter.

In a preferred embodiment, a taper angle α of each of the bumps 222 isless than or equal to 90 degrees. Each of the bumps 222 has a maximumheight H of about 0.5 μm to about 3 μm, and each of the bumps 222 hasbottom width W of about 0.5 μm to about 5 μm. Significantly, thecontours of bumps 222 dominant the shapes of the cavities formed in thefollowing steps, which are described in detail hereinafter.

In step 140, as shown in FIG. 2D, a first epitaxial layer 231 isepitaxially formed on the exposed portion of the substrate 210 such thatthe first epitaxial layer 231 covers a portion of each of the bumps 222,but a top surface 222 t of each of the bumps 222 is exposed out of thefirst epitaxial layer 231. In particular, when the first epitaxial layer231 is epitaxially grown, the first epitaxial layer 231 crawls along thesurface of the bump 222. The first epitaxial layer 231 would completelycovers the bumps 222 if the process of the epitaxial growth is notstopped. Accordingly, one feature of the present disclosure relays onthat the epitaxial growth of the first epitaxial layer 231 is soppedbefore the first epitaxial layer 231 completely covers the bumps 222 sothat the top surface 222 t of each of the bumps 222 is exposed out ofthe first epitaxial layer 231.

In one embodiment, the first epitaxial layer 231 includes a groupIII-nitride semiconductor, such as gallium nitride. The first epitaxiallayer 231 may be formed by techniques such as hydride vapor phaseepitaxy processes, metal organic chemical vapor deposition processes ormolecular beam epitaxy processes.

In step 150, the bumps 222 are removed by etching approaches to form aplurality of cavities 224 exposing a surface of the substrate 210, asdepicted in FIG. 2E. Since the top surface 222 t of each of the bumps222 is exposed out of the first epitaxial layer 231, the bumps 222 maybe removed by wet etching processes. For instance, the etchant may be amixed solution containing ammonium fluoride (NH₄F) and hydrogen fluoride(HF). Significantly, the position, volume and shape of the cavities 224substantially depend upon that of the bumps 222 as well as the coveragelevel that the first epitaxial layer 231 covers the bumps 222.Accordingly, the morphologies of the cavities 224 are controlled inadvance of step 140 because the shape and the volume of each of thebumps 222 as well as the arrangement of these bumps 222 are preciselycontrolled in step 130 by the process of patterning the sacrifice layer220, according to the embodiments of the present disclosure.

FIG. 2G is a top view schematically illustrating the cavities 224 formedin step 150 according to one embodiment of the present disclosure. Inthis embodiment, when the bumps 222 are removed, each of the cavities224 has an opening 224 a and a bottom portion 224 b, in which the areaof each of the openings 224 a is about 5% to about 50% of the area ofthe corresponding bottom portion 224 b, specifically about 15% to about40%. When the area of the opening 224 a is less than about 5% of thearea of the bottom portion 224 b, it is difficult to rapidly remove thebumps 222 in step 150. On the other hands, when the area of the opening224 a is greater than about 50% of the area of the bottom portion 224 b,the coverage of the first epitaxial layer 231 over the bumps 222 isinsufficient and is unfavorable to the subsequent step 160. Accordingly,the area of each of the opening 224 a is preferably about 5% to about50% of the area of the corresponding bottom portion 224 b, morepreferably about 15% to about 40%, according to the embodiments of thepresent disclosure.

In step 160, a second epitaxial layer 232 is epitaxially formed on thefirst epitaxial layer 231 such that the cavities 224 are enclosed by thefirst epitaxial layer 231 and the second epitaxial layer 232, as shownin FIG. 2F. In other words, by epitaxially forming the second epitaxiallayer 232, the cavities 224 become closed air voids embedded in thefirst epitaxial layer 231 and the second epitaxial layer 232. The methodof forming the second epitaxial layer 232 may be the same as thesedescribed hereinbefore in connection with the first epitaxial layer 231.In one embodiment, the second epitaxial layer 232 includes a group IIInitride semiconductor. In one example, the material of the secondepitaxial layer 232 may be the same as that of the first epitaxial layer231. For example, both the first and the second epitaxial layer 231, 232may be made of gallium nitride. In other embodiments, the refractiveindex of the second epitaxial layer 232 may be different from that ofthe first epitaxial layer 231 when taking the entire optical path intoconsideration.

In another embodiment, the horizontal growth rate in the growth of thesecond epitaxial layer 232 is greater than the horizontal growth rate inthe growth of the first epitaxial layer 231. The horizontal growth ratemay be controlled by the temperature and the pressure of the epitaxialgrowth process. In one example, a temperature of the second epitaxiallayer 232 in the growth of the second epitaxial layer 232 is greaterthan a temperature of the first epitaxial layer 231 in the growth of thefirst epitaxial layer 231, and a pressure of forming the secondepitaxial layer 232 is less than a pressure of forming the firstepitaxial layer 231. In other words, the first epitaxial layer 231 isepitaxially grown in an environment at a low temperature and under ahigh pressure such that the first epitaxial layer 231 has a betterthree-dimensional structure. As compared to the first epitaxial layer231, the second epitaxial layer 232 is epitaxially grown in anenvironment at a higher temperature and under a lower pressure, suchthat the formation of the second epitaxial layer 232 exhibits anexcellent planar characteristic and a rapid growth rate in a horizontaldirection.

As described hereinbefore in connection with step 130 and step 140, thecavities 224 may be formed in a variety of shapes by controlling thecontours and the cross-sectional shapes of the bumps 222. FIG. 3A toFIG. 3D are cross-sectional views illustrating the process steps of anepitaxial growth method according to another embodiment of the presentdisclosure. In the embodiment shown in FIG. 3A to FIG. 3D, a number ofbumps 222, each having a trapezoidal cross-section, are formed, as shownin FIG. 3A. Thereafter, a first epitaxial layer 231 is epitaxiallyformed to cover a portion of each of the bumps 222, as shown in FIG. 3B.Subsequently, the bumps 222 are removed to form a plurality of cavities224, as shown in FIG. 3C. And then, a second epitaxial layer 232 isepitaxially formed on the first epitaxial layer 231 to enclose thecavities 224 with trapezoidal cross-sections, as shown in FIG. 3D.Similarly, the embodiment shown in FIG. 3E to FIG. 3H, thecross-sections of the cavities 224 may be formed like stair.

FIG. 4A to 4F are cross-sectional views illustrating the process stepsof an epitaxial growth method according to still another embodiment ofthe present disclosure. In FIG. 4A, a substrate 210 is provided. In FIG.4B, a buffer layer 212 is formed on a surface of the substrate 210. InFIG. 4C, a plurality of bumps 222 are formed on the buffer layer 212.Thereafter, as shown in FIG. 4D, a first epitaxial layer 231 isepitaxially formed on the buffer layer 212. Subsequently, as shown inFIG. 4E, the bumps 222 are removed to form a plurality of the cavities224. And then, as shown in FIG. 3D, a second epitaxial layer 232 isepitaxially formed on the first epitaxial layer 231 to enclose thecavities 224.

FIGS. 5A and 5B are cross-sectional views illustrating the process stepsof an epitaxial growth method according to still another embodiment ofthe present disclosure. In this embodiment, step 110 to step 140 may bethe same as these described hereinbefore in connection with FIG. 2A toFIG. 2D. Thereafter, as shown in FIG. 5A, only a portion of each of thebumps 222 is removed in step 150, and another portion of each of thebumps 222 remains. In particular, the process of etching the bumps 222is stopped before the bumps 222 are completely removed so that a portion222 a of each of the bumps 222 is remained on the substrate.Subsequently, as shown in FIG. 5B, a second epitaxial layer 232 isepitaxially formed on the first epitaxial layer 231 in step 160 suchthat the cavities 224 are enclosed by the first and the second epitaxiallayers 231, 232. In other words, the remained portions 222 a of thebumps may be formed in the closed air voids that are embedded in thefirst and the second epitaxial layers 231, 232 according to thisembodiment. In one example, a refractive index of the remained portions222 a of the bumps is different from that of the first epitaxial layer231 and/or the second epitaxial layer 232, and therefore the remainedportions 222 a serves as an optical medium to change the optical pathand the optical characteristics of the structure shown in FIG. 2F.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims.

What is claimed is:
 1. An epitaxial structure, comprises: a substratehaving a surface; a first epitaxial layer disposed over the substrateand defining a plurality of curved air voids tapering away from thesubstrate and an opening over each of the curved air voids; and a secondepitaxial layer disposed on the first epitaxial layer and collectivelydefining the curved air voids in a shape of hemisphere with the surfaceand the first epitaxial layer.
 2. The epitaxial structure according toclaim 1, further comprises a buffer layer disposed between the substrateand the first epitaxial layer, and the bottom portion of the closed airvoid is formed on the buffer layer.
 3. The epitaxial structure accordingto claim 1, further comprises a patterned sacrifice layer disposed onthe substrate, and a top surface of the patterned sacrifice layer formsthe bottom portion of the closed air void.
 4. The epitaxial structureaccording to claim 1, wherein the closed air void has a height of about0.5 μm to about 3 μm, the bottom portion of the closed air void has awidth of about 0.5 μm to about 5 μm.
 5. The epitaxial structureaccording to claim 1, wherein the surface of the substrate defines abottom portion of each of the air voids, and the second epitaxial layerdefines a top portion of each of the air voids.
 6. The epitaxialstructure according to claim 5, wherein each of the openings encloses anarea accounting for 15-40% of an area of the bottom portion.